The present invention relates to a process and an apparatus for producing a hydrogen-rich synthesis gas, for example, an ammonia synthesis gas.
The process for producing ammonia from a hydrocarbon feed stream, such as natural gas, is, of course, well known. Thus, a mixture of the hydrocarbon feed gas and water in the form of steam is subjected to an endothermic catalytic reaction to yield carbon monoxide and hydrogen. This reaction is commonly referred to as primary reforming. It is then necessary to introduce nitrogen, which is typically done in the form of air, to produce the requisite ammonia synthesis gas by what is referred to as secondary reforming.
In prior commercial ammonia processes, the primary and secondary reforming steps have typically been carried out in separate reactors, and such process is quite suitable and satisfactory in plant situations where it is necessary or desirable to produce steam for other uses within the plant. Thus, in such processes, the hot reaction effluent from the secondary reforming operation is used to generate and/or to superheat steam, either for use otherwise within the ammonia process or for export.
In situations where the production of steam is not necessary, it is accordingly advantageous to utilize the heat available from the secondary reforming step for other purposes within the synthesis gas production process. One such use of the available heat from secondary reforming is to provide the heat necessary for primary reforming. The provision of a process and apparatus to achieve such use at a high level of efficiency is accordingly a principal objective of the present invention.
The production of ammonia, as well as other products such as methanol which are derived from hydrocarbons, has evolved in the last several years into a sophisticated state-of-the-art technology, in which cost effective improvements are essential but are exceptionally difficult to accomplish. In view of this, it is quite desirable to be able to achieve both primary reforming and secondary reforming in a single reactor, so that the overall cost of the production process can be reduced by elimination of expensive reactors and associated non-essential equipment.
There have been prior efforts to provide satisfactorily such reactors, but certain significant shortcomings have been encountered. For example, U.S. Pat. No. 3,751,228 describes a reactor in which the hot reformed gaseous product is removed from the bottom of the reactor, rather than utilized to provide heat for the reforming reaction. Instead, hot gas is introduced from outside the reactor to provide the necessary heat for the reforming step. A similar reactor is described in U.S. Pat. No. 4,127,289.
U.S. Pat. No. 4,071,330 describes a reactor which is positioned within a fired furnace and utilizes heat transfer from the furnace across the shell of the reactor, to provide the requisite heat for the endothermic reforming reaction. The shell is formed of a heat conducting material such as high nickel-chrome steel.
In U.S. Pat. No. 3,549,335, an autothermal reactor is illustrated and described, which includes an outer shell with an inner shell spaced therefrom to provide an annular passageway through which the hydrocarbon and steam mixture passes, through openings in the inner shell at the lower section of the reactor and through the primary reforming catalyst bed positioned outside of the tubes. The gas is thereafter brought into contact with the combustion reaction product and ultimately removed from the reactor. Such reaction process, utilizing atmospheric air for the combustion step, does not provide efficient utilization of the exothermic heat of reaction, as is highly desirable in today's sophisticated and competitive state of the art.